The development of solvent-extraction methods for determining trace amounts of uranium-233 in irradiated thorium is described. Thorium and its alpha-emitting daughters are complexed with EDTA, and, when uranium-233 is extracted as its diethyldithiocarbamate complex, only bismuth-212 accompanies it. This is immaterial for colorimetric or fluorimetric finishes, but, for determination of the uranium-233 by alpha counting, the bismuth-212 must first be allowed to decay. If, however, the uranium-233 is extracted as its 8-hydroxyquinoline complex, no alpha emitter accompanies it and concentrations of uranium-233 ranging from 100 p g per ml down to 0.01 pg per ml in 0-7 M thorium solution have been determined in this way.IN Part I of this series1 a method of determining uranium-233 in irradiated thorium was described, in which uranium-233 was separated from thorium daughters by solvent extraction with isobutyl methyl ketone (hexone). Separation from thorium itself was completed by forming the uraniumdiethyldithiocarbamate complex, which was retained in the solvent while the thorium was washed into the aqueous phase. It was thought that the method might be improved by the introduction of ethylenediaminetetra-acetic acid (EDTA) , since this does not prevent the solvent extraction of uranium as its diethyldithiocarbamate complex, whereas thorium and interfering daughter activities are preferentially complexed by EDTA.Unfortunately, as Bode2 has since reported, the bismuthdiethyldithiocarbamate complex is stronger than the bismuth-EDTA complex and, under the conditions for extracting uranium, the separated uranium-233 was found to be contaminated with alpha activity, owing to the presence of bismuth-212.The problem was to find a complexing agent that would form a strong solvent-soluble complex with uranyl ions, but any complexes formed with thorium and daughter alpha emitters must be weaker than their respective EDTA complexes. Di-and tri-92-butyl phosphates were tried, but were found to be unsatisfactory.Pfibil and Mal&t3 had reported that only uranium, titanium, vanadium, molybdenum and tungsten are precipitated by 8-hydroxyquinoline (oxine) in the presence of EDTA from an acetic acidammonium acetate buffer. Solvent extraction of the uranium complex
with eV-0.S}x and a>-1.6/*, one obtains 0.33, as compared with 0.14 for the Dirac electron, while with w=1.85ju the values are 0.16 and 0.03, respectively. Apart from this difference in magnitude, there is also a typical minimum of R with increasing incident energy below co = 3ju, which is impossible with the Dirac electron, where R drops montonically to zero for such high values of o>. Similar marked effects in the region of validity of the ordinary interpretation can be found in other more realistic problems (e.g., Coulomb and Compton scattering, details of which will be given elsewhere), and they may thus lead to tests of the theory regardless of questions of interpretation in other regions.
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